1. Field of the Invention
The invention relates to a manufacturing method of an attraction plate for electromagnetic door locks, particularly to one that manufactures an attraction plate with two convex-curve surfaces to form a curved internal stress for door locks.
2. Description of the Related Art
FIG. 1 is an armature for electromagnetic locks in U.S. Pat. No. 4,957,316 disclosed by Frolov. Frolov disclosed an electromagnetic lock having an electromagnet mounted to a door frame and having an armature 22 mounted to a door 10 for attraction to the electromagnet to lock the door 10. The armature 22 has a strike plate 29 and a back plate 30. The strike plate 29 further has a recess for engagement with an end of a bolt 25, and the back plate 30 is disposed between the strike plate 29 and the door 10 and is penetrated through by the bolt 25 to be mounted to the door 10. In addition, the armature 22 has a surface 39 at the reverse side of the side with the recess, and the center of the armature 22 is slightly bowed toward the surface 39 thereof within a range of 0.004-0.005 inches. Such structure allows the back plate 30 of the armature 22 to attach to the strike plate 29 in operation; however, the range of bowing between 0.004-0.005 inches does not apply to attraction plates with different thicknesses. According to experiments conducted by the inventor, when Frolov's invention is applied to attraction plates with different thickness, the results may not be satisfying. In other words, the thickness of attraction plates in electromagnetic locks has essential relation with bowing of armatures.
FIG. 2 is an improved attraction plate structure for electromagnetic door locks disclosed by the present inventor in U.S. Ser. No. 14/149,166 filed on Jan. 7, 2014, which has since issued as U.S. Pat. No. 9,341,007. It has an attraction plate 30 disposed on a mounted body 40 by a positioning assembly. The attraction plate 30 has a recessed portion 32 arranged on an attraction surface 31 at a central region thereof, and the recessed portion 32 further extends towards both ends 34 to form a concave-arc surface 33 on the attraction plate 30. The concave-arc surface 33 is arranged to be lower than a horizontal plane within a range of 0.06-0.26 mm to achieve strong tensile force for applications. This application has exactly an opposite structure to the present invention; nevertheless, it can also achieve a satisfying tensile value.
In fact, apart from different designs of the attraction plate, thickness of the attraction plate also has something to do with the tensile values in such structure. Without proper numbers of the thickness, the tensile values would not achieve the best numbers in operation. Therefore, thickness of the attraction plate shall be further disclosed in accordance with a height arranged at a center of the attraction plate to obtain stronger tensile values.